Efficient Heat Removal Research Articles

Vertically aligned graphene film/epoxy composites as heat dissipating materials

Efficient removal of heat has become one of the most critical challenges in the development of modern microelectronic devices. However, it is hard to dramatically improve thermal conductivity of composite materials even with highly loaded thermally conductive fillers due to the lack of efficient heat transfer paths. In this article, both horizontal and vertical graphene film/epoxy (GF/E) composites were designed and fabricated and their thermal and mechanical properties were studied. Vertically aligned structure constructed of continuous graphene films forms channels for heat removal. The thermal conductivity of vertical GF/E composites reaches 384.9 W m−1 K−1 at 44 vol% graphene, i.e. a dramatic enhancement of 3570% per vol% compared to the pure epoxy, and representing the highest value of all epoxy based composites. The cooling performance of light-emitting diode (LED) package with the application of GF/E composites is enhanced by reducing the temperature of LED lamp about 20.7 °C compared with the application of pure epoxy. The vertical GF/E composite proved to be a potential material for heat management of LED or other electronic devices.

A Multiscale Morphological Insight into Graphene Based Coatings for Pool Boiling Applications

ABSTRACTThe application of graphene for pool boiling is an attractive option to facilitate compaction and promote efficient heat removal from high power density devices. In this context, chemical characterization of the depositions achieved through commonly employed coating techniques are an important topic of discussion. A detailed structure-property relationship between the morphologies obtained on the mono and multilayered graphene coatings and their corresponding pool boiling performance quantified by the experimental critical heat fluxes is presented. Three different types of graphene (G) and graphene oxide (GO) substrates are characterized: (i) nanoscale: mono and multilayer samples developed through chemical vapor deposition, (ii) Rochester Institute of Technology (RIT)-G/GO colloid generated through an oxygen embrittlement electrochemical process, and (iii) commercially available chemical vapor deposited (CVD)-G/GO colloid. The morphological features were characterized with scanning electron microscope while X-Ray Diffractometer analysis and Raman spectroscopy were used to examine the ordering and stacking of the sheets that result in the unique structural features. Fourier transform infrared and energy dispersive X-ray spectroscopy were employed to identify the overall compositional characteristics of the coated surfaces. The wettability changes and additional nucleation sites for nanoscale coatings, and multiscale roughness features and ridge microstructures for microscale coatings were identified as enhancement mechanisms.

Highly Productive Continuous Flow Synthesis of Di- and Tripeptides in Water

The reaction of amino acid derived N-carboxyanhydrides (NCAs) with unprotected amino acids under carefully controlled aqueous continuous flow conditions realized the formation of a range of di- and tripeptide products in 60–85% conversion at productivities of up to 535 g·L–1h–1. This required a fundamental understanding of the physicochemical aspects of the reaction resulting in the design of a custom-made continuous stirred tank reactor (CSTR) with continuous solids addition, high shear mixing, automated pH control to avoid the use of buffer, and efficient heat removal to control the reaction at 1 ± 1 °C.

High performance terahertz metasurface quantum-cascade VECSEL with an intra-cryostat cavity

A terahertz quantum-cascade (QC) vertical-external-cavity surface-emitting-laser (VECSEL) is demonstrated with over 5 mW power in continuous-wave and single-mode operation above 77 K, in combination with a near-Gaussian beam pattern with a full-width half-max divergence as narrow as ∼5° × 5°, with no evidence of thermal lensing. This is realized by creating an intra-cryostat VECSEL cavity to reduce the cavity loss and designing an active focusing metasurface reflector with low power dissipation for efficient heat removal. Also, the intra-cryostat configuration allows the evaluation of QC-VECSEL operation vs. temperature, showing a maximum pulsed mode operating temperature of 129 K. While the threshold current density in the QC-VECSEL is higher compared to that in a conventional edge-emitting metal-metal waveguide QC-laser, the beam quality, slope efficiency, maximum power, and thermal resistance are all significantly improved.

Abstract: Directional Freezing and Vitrification of Whole Limbs for Future Transplantation and Organ Banking

INTRODUCTION: According to the World Health Organisation, less than 10% of humanities needs for transplantable organs are being met. No data is available for vascularized composite tissue allotransplantations (VCA), yet these cases are further complicated by the need for an instantly available and compatible recipient. VCA containing skin, fat, blood vessels, bone, bone marrow and nerve are the ultimate tool available to date in reconstructive surgery. Widespread use of this tool is hindered in part by the availability of limbs which can survive a limited amount of time disconnected from the body. Cryobiology is the scientific field which investigates biology at low temperatures. Cryobiologists have long sought to cryopreserve biological samples ranging from single cells to complex organs and whole animals. Vitrification is the transformation of a substance into a non-crystalline amorphous solid using rapid heat removal. Directional freezing is a technique, consisting of highly controlled and cell-friendly ice crystal morphology that significantly reduces mechanical damage. Extremely efficient heat removal and controlled ice crystal propagation make it suitable for freezing both small and large volumes alike. Combining Directional freezing and VCA may open the door for “Organ Banking”. If composite tissue could be frozen and then thawed without damage, non transplanted tissue and organ waste would be reduced, potentially enabling better donor and recipient availability and match. METHODS: We used directional freezing and vitrification on a syngeneic heterotrophic rat hindlimb transplant model and monitored the animals and transplanted limbs for up to 72 hours. Cell and tissue samples were taken for culture and histology. RESULTS: Immediately upon thawing the donor hindlimb blood vessels and tissue felt similar to the recipient’s. Following revascularization reperfusion was clinically evident by color and bleeding from distal sites to the anastomosis. Limb survival was noted up to 72 hours post op. Tissue samples including muscle, skin and blood vessels where taken at 24, 48 and 72 hours from transplantation. Histology demonstrated viable myocytes, intact intimal lining of blood vessels and all skin layers. CONCLUSION: Whole limb freezing and reimplantation using directional freezing and vitrification is feasible in small animal models. Based on our current achievements we intend to develop protocols for limb cryopreservation in large animals, aiming to advance the field of cryobiology towards the ambitious goal of human organ banking.

Heat removal and catalyst deactivation in a Sabatier reactor for chemical fixation of CO2: Simulation-based analysis

Thermo-catalytic hydrogenation of CO2 into synthetic CH4 via Sabatier reaction is an attractive route to reduce fossil fuels consumption and to limit greenhouse gas emissions, while potentially providing revenue. Hydrogen required for the reaction can be generated by water electrolysis using renewable or low carbon footprint electricity. With respect to CO2 methanation, a number of technological challenges have to be resolved in order to make this technology economically viable. The highly exothermic nature of the Sabatier reaction makes heat removal a challenging task. Another major problem is catalyst deactivation by coking caused by CH4 cracking at elevated temperatures. Maximizing CH4 production over extended periods of operation will require highly efficient heat removal to facilitate CH4 production, while minimizing catalyst deactivation. In this study, a heat-exchanger type, molten salt-cooled packed bed reactor is analyzed using a transient mathematical model. The model considers inter-compartment heat exchange and catalyst deactivation by coking, assuming the use of a Ni/Al2O3 catalyst. The reactor performance was investigated in terms of CO2 conversion and CH4 yield for the case of the feed containing CO2 and H2, and for the case of the reactor fed with biogas (40% CO2 and 60% CH4) and H2. The model predicts that, though the catalyst deactivation leads to a substantial decline in the reactor performance, CH4 yields and CO2 conversions over 80% are achievable after 10,000h of operation.

Thermal Properties of Graphene Filled Polymer Composite Thermal Interface Materials

Recent years have witnessed a staggering escalation in the power density of modern electronic devices. Because increasingly high power density accumulates heat, efficient heat removal has become a critical limitation for the performance, reliability, and further development of modern electronic devices. Thermal interface materials (TIMs) are widely employed between the two solid contact surfaces of heat sources and heat sinks to increase heat removal for electric devices. Composites of graphene and matrix materials are expected to be the most promising TIMs because of the remarkable thermal conductivity of graphene. Here, the recent research on the thermal properties of graphene filled polymer composite TIMs is reviewed. First, the composition of graphene filled polymer composite TIMs is introduced. Then, the synthetic methods for graphene filled polymer composite TIMs are primarily described. This study focuses on introducing the methods for improving and characterizing the thermal properties of graphene filled polymer composite TIMs. Furthermore, the challenges facing graphene filled poly­mer composite TIMs for thermal management applications in the modern electronic industry and the further progress required in this field are discussed. image

Design of novel geometries for microchannel heat sinks used for cooling diode lasers

High power diode laser bars require thermal management packages with high cooling efficiencies and long-term stability. At a constant size, the power output of the diode laser is being increased rapidly; therefore, an efficient heat removal strategy has become even more important. High efficiency microchannel heat sinks are usually used to remove heat from the system. In this study, the fluid flow and heat transfer of an existing diode laser micro-channel heat sink is numerically simulated. To reduce the maximum temperature and obtain a uniform temperature distribution across the diode laser, which leads to extended lifetimes, the simulations are repeated for three different channel geometries. The hydrodynamic and thermal behavior of the heat sinks are then compared. The results show that among all geometries, the sinusoidal structure provides the best thermal performance. Although the system pressure drop increases by 18%, this configuration increases the diode lifetime by 44%.

Statistics of localized phase slips in tunable width planar point contacts

The main dissipation mechanism in superconducting nanowires arises from phase slips. Thus far, most of the studies focus on long nanowires where coexisting events appear randomly along the nanowire. In the present work we investigate highly confined phase slips at the contact point of two superconducting leads. Profiting from the high current crowding at this spot, we are able to shrink in-situ the nanoconstriction. This procedure allows us to investigate, in the very same sample, thermally activated phase slips and the probability density function of the switching current Isw needed to trigger an avalanche of events. Furthermore, for an applied current larger than Isw, we unveil the existence of two distinct thermal regimes. One corresponding to efficient heat removal where the constriction and bath temperatures remain close to each other, and another one in which the constriction temperature can be substantially larger than the bath temperature leading to the formation of a hot spot. Considering that the switching current distribution depends on the exact thermal properties of the sample, the identification of different thermal regimes is of utmost importance for properly interpreting the dissipation mechanisms in narrow point contacts.

Commissioning and experimental validation of SST-1 plasma facing components

Plasma facing components of SST-1 are designed to withstand an input heat load of 1.0 MW/m2. They protect vacuum vessel, auxiliary heating source i.e. RF antennas, NBI and other in-vessel diagnostic from the plasma particles and high radiative heat loads. PFC’s are positioned symmetric to mid-plane to accommodate with circular, single and double null configuration. Graphite is used as plasma facing material, back made of copper alloy and SS cooling/baking tubes are brazed on copper alloy back plates for efficient heat removal of incident heat flux. Benchmarking of PFC assembly was first carried out in prototype vacuum vessel of SST-1 to develop understanding and methodology of co-ordinate measurements. Based on such hands-on-experience, the final assembly of PFC’s in vacuum vessel of SST-1 was carried out. Initially, PFC’s are to be baked at 250 °C for wall conditioning followed with cooling for heat removal of incident heat flux during long pulse plasma operation. For this purpose, the supply and return headers are designed and installed inside the vacuum vessel in such a way that it will cater water as well as hot nitrogen gas depending up on the cycle. This paper will discuss the successful installation of PFC’s and its plasma operation respecting all design criteria.

Thermal conduction across a boron nitride and SiO2 interface

The need for efficient heat removal and superior thermal conduction in nano/micro devices has triggered tremendous studies in low-dimensional materials with high thermal conductivity. Hexagonal boron nitride (h-BN) is believed to be one of the candidates for thermal management and heat dissipation due to its novel physical properties, i.e. as a thermal conductor and electrical insulator. Here we report the interfacial thermal resistance between few-layer h-BN and its SiO2 substrate using the differential 3ω method. The measured interfacial thermal resistance is around ~1.6 × 10−8 m2K W−1 for monolayer h-BN and ~3.4 × 10−8 m2K W−1 for 12.8 nm-thick h-BN in metal/h-BN/SiO2 interfaces. Our results suggest that the voids and gaps between the substrate and thick h-BN flakes limit the interfacial thermal conduction. This work provides a deeper understanding of utilizing h-BN flakes as a lateral heat spreader in electronic and optoelectronic nano/micro devices with further miniaturization and integration.

THERMOELECTRIC HEAT REMOVAL SYSTEM FOR THE OPERATIONAL STABILISATION OF HEAT PIPES IN A SYSTEM FOR PROVIDING THE THERMAL REGIME OF RADIO ELECTRONIC EQUIPMENT

Objectives . The aim of the study is to conduct an analysis of thermophysical processes in a thermoelectric system used for providing the thermal regime of electronic equipment located in a cabinet. A cabinet design and thermoelectric system for efficient heat removal from the condensing part of the heat pipe are proposed. An additional advantage of the proposed design is the obviation of significant additional power consumption requirement for regulating the temperature of radio electronic equipment stored in the cabinet.A distinctive feature of the constructive realisation is the presence of an intermediate heat removal. Methods. The three-dimensionality of the problem and mixed boundary conditions lead to the need to develop a calculation of heat transfer in the elements of the construction of the thermoelectric system. The numerical calculation method is based on the method of energy balances. The analysis of the heat regimes of the intermediate heat removal is performed on the basis of a mathematical model for a locally-heated and -cooled restricted plate. Results. A cabinet design and a thermoelectric system for efficient heat removal from the condensing part of the heat pipe are proposed. A distinctive feature of the constructive realisation is the presence of an intermediate heat removal. Conclusion. The capacity of the intermediate heat removal for given dimensions and temperature of the source is weakly affected by its thickness (in constructively reasonable limits), as well as the temperature and area of the absorbing side of the thermoelectric module; the total heat output from the heat source is determined by the dimensions and heat exchange conditions on the free surface of the intermediate heat removal, as well as by the temperature and dimensions of the heat absorbing side of the thermoelectric module. The use of an intermediate heat removal can significantly reduce the thermal load on the thermoelectric module with a slight decrease in the temperature driving force.

Machining comparison of aerospace materials considering minimum quantity cutting fluid: A clean and green approach

Efficient removal of heat from the chip formation zone in the machining of aerospace materials is quite crucial for attaining viability with respect to cost and productivity. The recently embraced cooling and lubrication method in the context of environmental friendly and sustainable manufacturing includes the application of minimum quantity cutting fluid. This article presents an experimental investigation, complemented with an evolutionary optimization technique, for studying the impact on cutting forces, surface roughness, tool wear, and chip control in the turning of the two aero-engine alloys (Inconel-800 and titanium-II) with or without using minimal-quantity cooling lubrication fluid. In addition, the multiple regression technique is applied to find the relationship between responses and input parameter such as cutting speed, feed rate, and approach angle. Afterward, the sensitivity analysis and analysis of variance (ANOVA) tests have been performed to test the statistical significance of proposed predictive models. At the end of work, the experimental data have been optimized through two evolutionary techniques, i.e. particle swarm optimization and bacterial foraging optimization, also compared to the much-used desirability technique. It has been concluded that the cooling option of applying minimum quantity cutting fluid proved beneficial for machining these aerospace materials. Moreover, the evolutionary techniques gave much more accurate results when compared to the desirability technique with particle swarm optimization and was concluded as the best one out of the three techniques on the basis of minimum average time taken and minimum percentage error.

Calculation of temperature profiles in solid-state laser disks mounted on multi-layered heat spreaders using Hankel transforms.

The main challenge in disk laser design is the realization of efficient heat removal from the pumped area by optimizing the heat spreader design and the water impingement cooling. This generally requires the calculation of the temperature distribution in the disk by numerically solving the heat conduction equation using finite element algorithms. We have developed a simple method to calculate disk temperature profiles that is based on analytically solving the heat conduction equation in Hankel transform space. This method can be applied to disks that are mounted on multi-layered, water-cooled heat spreaders, which may include glue or solder layers and dielectric coating layers. The temperature and heat fluxes at the interfaces of the layers are connected via a heat transfer matrix, which allows for straightforward incorporation of additional heat sink layers or an undoped cap into the model. This generalized model allows for the parametric optimization of the heat distribution in pumped solid-state laser and semiconductor laser disks.

Enhanced Percolating Thermal Underfills Achieved by Means of Nanoparticle Bridging Necks

Efficient heat removal from integrated circuits arranged vertically in 3-D chip stacks requires thermally conductive underfill materials. The low-heat-transport performance of traditional capillary underfills can be improved by percolating the thermal conductive filler particles. We increased the thermal path by adding quasi-areal contacts using nanoparticle assemblies directed to the contact points of the percolating filler particles. We studied the formation and thermal effect of such nanoparticle neck assemblies in the filler-particle contact points using aqueous suspensions containing nanoparticles of different sizes, size distributions, and concentrations. An optimized binary mixture of small (28–43 nm) and large (200–300 nm) nanoparticles results in dense and defect-free neck assemblies. A neck-enhanced percolating thermal underfill (PTU) with a thermal conductivity as high as 2.4 W/mK was achieved using alumina filler and nanoparticles. Compared to a PTU, the addition of nanoparticle necks resulted in a more than twofold improvement in thermal conductivity.

Cooling Geometry Dependent Operation Properties in a Diode End-Pumped Monolithic Yb:YAG Laser

Operation properties, such as output power, beam radius, and crystal temperature rise were investigated in a diode-pumped edge-cooled monolithic Yb:YAG laser with ~ 4 W pumping level. The output power showed saturation behavior because of severe temperature rise and poor spatial overlapping between pump beam and lasing mode. Face cooling geometry by using a sapphire plate was also investigated. No apparent saturation behavior in output power was observed due to the reduced temperature rise, which could be attributed to efficient heat removal through the crystal-sapphire interface.

Experimental investigation on natural convection heat transfer characteristics of C-shape heating rods bundle used in PRHR HX

The heat transfer performance of Passive Residual Heat Removal Heat Exchanger (PRHR HX) immerged in the In-containment Refueling Water Storage Tank (IRWST) is of great importance for the efficient and safe removal of the residual heat in the AP1000 plant. The C-shape heating rods bundle is being used in the PRHR HX, yet the heat transfer characteristics of this special C-shape heat exchanger are not completely clear. The Westinghouse Electric Corporation has performed the PRHR HX experiment using three vertical tubes to validate the design of PRHR HX. However, the horizontal sections of the C-shaped bundle were neglected, and attentions were mainly focused on the local heat transfer effect of the vertical tubes. In the present work, an overall scaled-down IRWST and PRHR HX models are built to simulate the thermal hydraulic process in the residual heat removal accident. The natural convection heat transfer characteristics of both the overall IRWST model and the local C-shaped PRHR HX bundle model are researched in the first stage. For the overall IRWST model, the natural convection flow vectors are qualitatively measured by PIV technique. The velocity and temperature distributions are influenced mutually, then thermal stratification phenomenon develops in the overall IRWST. The existence of the “thermal interface” has great influences on the thermal stratification. For the local PRHR HX heat transfer, traditional natural convection correlations for both horizontal and vertical bundles are compared to the experimental data. For the vertical bundle of the PRHR HX model, a revised Rayleigh number is proposed as a criterion for the slender vertical rod bundles, when the vertical rod bundle outside flow cannot be treated as flat plate. Then revised correlations are proposed to predict the PRHR HX average heat transfer coefficient. Moreover, the analysis shows that the forced convection also plays an important role in enhancing the heat transfer effect of the PRHR HX model upper horizontal bundle, so the modified Numix is recommended for the PRHR HX upper horizontal bundle secondary heat transfer calculation.

Vertically Aligned and Interconnected Graphene Networks for High Thermal Conductivity of Epoxy Composites with Ultralow Loading

Efficient removal of heat via thermal interface materials has become one of the most critical challenges in the development of modern microelectronic devices. However, traditional polymer composites present limited thermal conductivity even when highly loaded with highly thermally conductive fillers due to the lack of efficient heat transfer channels. In this work, vertically aligned and interconnected graphene networks are first used as the filler, which is prepared by a controlled three-step procedure: formation of graphene oxide liquid crystals, oriented freeze casting, and high-temperature annealing reduction under Ar. The obtained composite, at an ultralow graphene loading of 0.92 vol %, exhibits a high thermal conductivity (2.13 W m–1 K–1) that is equivalent to a dramatic enhancement of 1231% compared to the pure matrix. Furthermore, the composite also presents a much reduced coefficient of thermal expansion (∼37.4 ppm K–1) and increased glass transition temperature (135.4 °C). This strategy provide...

An approach towards a perfect thermal diffuser.

A method for the most efficient removal of heat, through an anisotropic composite, is proposed. It is shown that a rational placement of constituent materials, in the radial and the azimuthal directions, at a given point in the composite yields a uniform temperature distribution in spherical diffusers. Such arrangement is accompanied by a very significant reduction of the source temperature, in principle, to infinitesimally above the ambient temperature and forms the basis for the design of a perfect thermal diffuser with maximal heat dissipation. Orders of magnitude enhanced performance, compared to that obtained through the use of a diffuser constituted from a single material with isotropic thermal conductivity has been observed and the analytical principles underlying the design were validated through extensive computational simulations.

Floquet topological systems in the vicinity of band crossings: Reservoir-induced coherence and steady-state entropy production

Results are presented for an open Floquet topological system represented by Dirac fermions coupled to a circularly polarized laser and an external reservoir. It is shown that when the separation between quasi-energy bands becomes small, and comparable to the coupling strength to the reservoir, the reduced density matrix in the Floquet basis, even at steady-state, has non-zero off-diagonal elements, with the magnitude of the off-diagonal elements increasing with the strength of the coupling to the reservoir. In contrast, the coupling to the reservoir only weakly affects the diagonal elements, hence inducing an effective coherence. The steady-state reduced density matrix synchronizes with the periodic drive, and a Fourier analysis allows the extraction of the occupation probabilities of the Floquet quasi-energy levels. The lack of detailed balance at steady-state is quantified in terms of an entropy production rate, and it is shown that this equals the heat current flowing out of the system, and into the reservoir. It is also shown that the entropy production rate mainly depends on the off-diagonal components of the Floquet density matrix. Thus a stronger coupling to the reservoir leads to an enhanced entropy production rate, implying a more efficient removal of heat from the system, which in turn helps the system maintain coherence. Analytic expressions in the vicinity of the Dirac point are derived which highlights these results, and also indicates how the reservoir may be engineered to enhance the coherence of the system.